Caption: Euglena flagellate protozoan. Computer artwork of the flagellate protozoan Euglena gracilis. This freshwater single-celled organism can either obtain energy from sunlight, via photosynthesis, or by absorbing chemicals from its surroundings. Cell organelles called chloroplasts (green) are used for photosynthesis. A sensory organ, known as the eye spot, or stigma (pink), is able to perceive the intensity of light. This allows E. gracilis to move, using its tail-like flagellum (top), to the areas of most intense light. Also seen is the nucleus (purple), which contains the cell's genetic information.